Identification of potato genes involved in Phytophthora infestans resistance by transposon mutagenesis

The late blight disease, caused by the oomycete Phytophthora infestans (Mont.) de Bary, is a serious threat to the potato crop every growing season. This has, for example, led to the disastrous Irish famine in the middle of the 19 th century, and continued in the 20 th century to remain a serious problem for potato growers. Since the early 1980s P. infestans populations changed more rapidly and epidemics even increased in their severity. Resistance breeding stimulated the introduction of resistance genes ( R genes) from wild Solanum species into cultivated potato, Solanum tuberosum , but newly occurring virulent races of P. infestans circumvented these R gene mediated resistances and no cultivars with durable resistance were obtained. At the moment, methods using fungicides supervised by spraying control via decision support systems are the only available control measures.Characteristic for R gene type mediated resistance reactions is the hypersensitive response (HR) leading to local cell death causing necrotic spots at the site of attempted infection. Genetic analysis of HR mediated resistances showed that activation of HR is highly specific and induced upon recognition between a specific R gene in the plant and a corresponding avirulence gene ( Avr gene) in the pathogen. Insights in the molecular mechanisms underlying this HR resistance reaction in Solanum species might facilitate the development of potato cultivars that are more durable in maintaining a resistance phenotype.A two component Ac-Ds transposon tagging strategy in diploidised potato was developed to identify and isolate genes involved in the R1 gene mediated resistance response to P. infestans . Transposable elements are molecular genetic tools to mutate and identify genes. The transposable elements Ac and Ds were first characterised in maize and their molecular isolation led to the identification of maize genes that were tagged by these elements. The autonomous Ac element is able to transpose by itself and also to induce transposition of the non-autonomous Ds element that is transposase defective. Introduction of these elements in heterologous species demonstrated their utility for isolating genes in self-fertilising plant species. Also in the highly heterozygous and tetraploid potato, the Ac and Ds transposable elements were shown to be functional. A cell autonomous visual marker gene for potato, the granule bound starch synthase gene ( GBSS gene), enabled a refined characterisation of Ac transposition in potato. Further molecular characterisation showed high levels of Ac-Ds transposition both somatically and germinally, so that suitable populations could be generated for tagging purposes.The production of clones homozygous for the gene of interest that are normally required for efficient tagging strategies, turned out to be time consuming in potato due to self-incompatibility at the diploid level. Therefore, an alternative method based on somatic transposition was developed for the direct selection of transposition events instead of recovering germinally transmitted transpositions. Highly chimaeric Ac-Ds seedlings with active Ds transposition linked to the R1 resistance gene on chromosome 5 of potato were selected. Protoplasts were isolated from actively transposing seedlings and using the hygromycin excision selection marker, regenerants could be selected with new independent Ds insertions. The resulting R1 resistant transposon mutagenised population of almost 2000 hygromycin resistant regenerants formed an ideal start for the identification of an R1 tagged mutant, or other Ds insertional mutants with an altered R1 resistance response.The somatically regenerated tagging population was analysed for the P. infestansR1 type HR resistance response, using a detached leaf assay for P.infestans inoculation. In a primary screening, 33 potential R1 resistance variants showing partial susceptibility to P. infestans race 0 were identified. These results were further quantified using stringent inoculation conditions on replicate samples leading to the identification of four putative mutants with a distinctly altered R1 resistance response. In these putative mutants less than 50% of the inoculated leaves showed the R1 type HR response and clear colonisation with sporulation of P. infestans was observed. The flanking sequences of the Ds insertion sites in these putative R1 mutants were analysed and in two cases a potential biological correlation between the insertion sequences and the phenotype was evident. One putative mutant contained a Ds insertion in a region with auxin and abscisic acid response cis-elements homologous to a specific region (TAPIR) of the tomato defence related genes TAPG2 and TAP1 .The second P. infestansR1 resistance mutant, mutant 1000 with a striking susceptible phenotype was characterised in more detail. Two Ds insertions were identified and the insertion site flanking sequences both showed high homology to serine/threonine protein kinases. The Ds insertion sites turned out to be homologous but not identical, indicating two independent Ds insertions in homologous but not identical genes. Both sequences showed protein identity to all the conserved regions of serine/threonine protein kinases and they contained a conserved intron position. The closest homology was to the serine/threonine protein kinase domain of the Xanthomonas resistance gene Xa21 , which is involved in the induction of a HR resistance response in rice. This indicates that the isolated Solanum tuberosum protein kinase (StPK) homologs are candidate genes involved in resistance gene activity in potato. Further specific molecular analyses identified at least 11 homologs by sequence, which probably belong to a large family of serine/threonine protein kinases in potato. Both homologs in which the Ds transposons are inserted were present in susceptible parental potato clones. Therefore, it is unlikely that the isolated sequences represent the R1 gene itself. The mutated StPKs were designated rpr1 and rpr2 , r equired for P hytophthora infestansr esistance gene 1 and 2. Studying these mutants and the StPKs involved might help in understanding the pathway leading to HR resistance in potato.